ASSIGNMENT_V

1) The acronym SCSI stands for Small Computer System

Interface. It refers to a standard bus defined by the American Natinalstandards institute(ANSI) Under the designation X3.131[2].

2) In the original specifications of the standard, devices such as

disks are connected to a computer via a 50- wire cable,which can be up to 25 metersin length and can transfer data rates up to 5 megabytes.

3) A SCSI bus may have 8 datalines,in which case it is called a

narrow bus transfers data one byte at a time. The SCSI connectormay have 50,68,or 80 pins ,because SCSIhas various options.

4) Transfer Rate Of SCSI Bus :

The maximum transfer rate in commercial devices that are

currently available varies from 5 megabytes to 160 megabytes.The most recently version ofthe standard is intended to support transfer rates up to 320 mega ytes , and 640 megabytes.

The maximum transfer rate on a given bus is often a function of

the cable and the number of devices connected ,with higher rates for a shorter cable andfewer devices.

5) To achieve the high data transfer rate ,the bus length is

typically limited to 1.6 m for SE signaling and 12 m for LVd signaling. The maximum capacityof the SCSI Bus is 8 devices for a narrow bus, and 16 devices for a wide bus.6) The SCSI bus is connected to the processor bus through SCSI controller , as shown in the below figure.

Main Processor Memory

Bridge

Additional SCSI Ethernet Usb

ISA interface memory controller interface controller

IDE Disk Video Disk cd-Rom controller controller

CD-Rom Key board

Disk1 Disk2 GameAn Example of a computer System using different interfacestandards

7) This controller uses DMA to transfer data packets from the

main memory to the device ,or vice versa. A packet may contain a block of data, commands from theprocessor to the device , or status information about the device.

8) Operation Of the SCSI Bus :

To know the operation of the SCSI Bus , let us consider how it

may be used with a disk drive.The communication with adisk drive differs substantiallyfrom the communication with the main memory .

Data are stored on a disk in blocks.called sectors .where each

sectors may contain several undred bytes.These data may not be necessarily bestored in contigious sectors. Some secors may be defective and must be skipped.

9) In disk drive a read or write request may result in accessing

several constraints of the mechanical motion of the disk, there is a long delay on theorder of the everal milliseconds, before reaching the first sector to or from whichdata are to be transffered. Then a burst of data are transffere at high speed .Anotherdelay may ensue ,follwed by aburst data.A single read/write request may involve severalsuch bursts .Hence the SCSI protocol is designed to facilitate this mode of operation.10) A controller connected to a SCSI bus is one of two types aninitiator or a target .An initator has the ability to select a particular targer and sendcommands specifying the operations to be performed. The disk controllers operates asa target . It carries out the commands it receive from the initator.

The intiator establishes a logical connection has been

established it can be suspended and restored as needed to transfer commands and bursts ofdata while particular connection is suspended,other devices can be use the bus totransfer information .

11) This ability to overlap data transfer request is one of the keyfeatures of the SCSI bus that leads to it’s high performance.

12) Data transfers on the SCSI bus are always controlled by thetarget controller. To send a command to a target, an initiator requestcontroller of the bus and after winning arbitration , selects the controller.It wants tocommunicate with and hands control of the bus over to it.Then the controller starts a datatransfer pperation to receive a command from the initiator.

Phase -BSY Busy: asserted when the bus

Information type -C/D control/data:

Handshake -REQ request: asserted by atarget

to Request a data transfercycle.

-ACk asserted by the

initiator when it has completed aoperation

Direction of transfer –I/O input/output:

assrted to Indiacate an i/poperation.

14) Here _ sign indicates that the signals are active ,or thatdataline is equal to 1,when they are in the low voltage state. The bus has noaddresslines,Instead ,the datalines are used to identify the bus controllers involved duringthe selection or reselection and arbitration.

15) Phases Of SCSI Bus:

The main phases involved in the operation of the SCSI bus

are arbitration , selection,information transfer,and reselection.

Arbitration:

The bus is free when the _BSY signal is in the inactive state.Any controller can request the use of the bus while it is in this state.When 2 or morecotrollers may generate such a request at same time ,an arbitration scheme must beimplemented. A controller requests the bus by asserting the _BSY signal. And by assrting its associated dataline to identify itself .TheSCSI bus uses a simple distributed arbitration scheme. It is shown in below figure. ____ DB2

____ DB5

____ DB6

____ BSY

____ SEL

free Aribitration Selection

Phases involved in the operations ofthr SCSI bus

16) Assume that controller 6 is an initiator that establishes a

connection Controller 5 . After winning arbitration controller 6 proceeds to the selectionphase, in which identifies the target.

17) Selection:

Having won arbitration ,controller 6 contiues to assert _BSY

and _DB6 .Any other controller that may have been involved in the arbitrationphase ,such as controller 2 in the figure must stop driving the data lines. Once the _SEL line becomes active, if it has not already doneso.After placing the address of the target controller on the bus ,initiatorreleasest he _BSY signal.

18) The selected target controller responds by asserting –BSY

this informs the initiator that the connection it is requesting has beenestablished,so thatit may remove the address information from the datalines .The selection process is nowcomplete and the target controller is asserting _BSY signal.

19) Information Trarnsfer:

The information transfer between two controllersmay

consist of commands from the target to the initiator to the target states , responses fromthe target to the initiator or data being transferred to or from the i/o device .Handshake signaling is used to control Information transfers at the end of the transfer , the targetcontroller releases –BSY signal ,thus freeing the bus for use by other devices.

20) Reselection:

When a logical connection suspended and the target is

ready to be restore it,the target must first gain control of the bus .It’s starts an arbitrationcycle ,and after winnig arbitration ,it selects the initiator controller.The initiator nowasserting –BSY .Before data transfer begins,the initiator must hand controller overto the target . This is achieved by having target controller assert –BSY afterselecting the inititor the target respose with states information and data transferoperations .The latter are controlled by the target , because it is a target that knows when datais available ,when to suspend and reestablish connections Circuit for SCSI Bus